In prior art valve train mechanisms for internal combustion engines, either spark-ignited (SI) or compression-ignited (CI), mechanically operated poppet valves are typically employed, being actuated by a rotary cam and associated linkage known in the art as a valve train. Many arrangements of cam form, cam drive, and cam-to-valve linkage have been proposed and reduced to practice over the years. Some of the most popular have been codified in the art as Type-1, Type-2, etc. through Type-5.
One of the constraints common to prior art mechanically-actuated valve trains is that the spatial relationship between the cam and its associated valve or valves must observe well-defined geometric rules governing the kinematic behavior of the mechanism. One such constraint is that the cam follower must follow a path that is square to the cam surface (orthogonal of the cam axis of rotation) so that point loading at the interface is avoided. Similarly, a rocker that is in contact with a valve must describe an arc that falls in the same plane as the motion of the valve itself. If these basic mechanical rules are not observed, excessive noise and wear will result. Consequently, in the prior art the locations and motion paths of valves and camshafts are constrained within well-defined limits.
Several alternative mechanisms have been proposed that provide either cam-based lost-motion valve actuation, or in some cases, a cam-less mechanism. See, for example, U.S. Pat. No. 4,716,863 issued Jan. 5, 1988 to Pruzan; see also U.S. Pat. No. 6,227,154 B1 issued May 8, 2001 to Wakeman; see also U.S. Patent Application Publication No. U.S. 2003/0221663 A1, published Dec. 4, 2003, by Vanderpoel et al. With a cam-less mechanism certainly, and with specific embodiments of the lost-motion mechanism, spatial constraints of valve location relative to the camshaft no longer apply, thus permitting greater architectural freedom in design.
Given this freedom, there are several important benefits available to an engine design that is free of the constraints of a purely mechanical valve train:
1. Both SI and CI engines are known to benefit from provision of greater valve flow area and low port restriction, for both inlet valves and exhaust valves. These factors affect volumetric efficiency which in turn influences specific power and fuel consumption. To obtain these benefits, engine designs historically have evolved from two valves to four valves per cylinder, and in some cases to five valves per cylinder. Adding valves beyond four per cylinder is probably not cost-effective in a light-duty automotive engine, but a known way to obtain the benefits recited above with four valves is to move to radially-disposed valves wherein the valve axes are non-parallel to the cylinder axis and may or may not intersect at a point within the cylinder or engine head. This architecture allows larger valves to be specified for a given bore size, and permits straighter, less restrictive port designs. A very few prior art production engines have employed radial valves, a severe problem being the expensive and complex mechanism necessary to address the linkage issues. If such constraints are overcome, then the benefits of a radial valve layout are open to the engine designer. Thus, what is needed in the art is a simplified means for obviating the restraints of prior art mechanical valve trains.
2. A current trend in diesel engine combustion systems is a shift from conventional diffusion combustion toward a partially pre-mixed combustion mode in which a portion of the fuel charge is injected early during the compression stroke rather than late in the stroke near top dead center (TDC) as in the older prior art. When early injection is attempted with conventional nozzles optimized for late injection and having an included spray angle of about 150°, there is a high probability of fuel's impinging undesirably on the cylinder walls, leading to premature engine wear. This, in turn, is driving a further trend toward narrower angle spray patterns in an attempt to obtain a long free-plume length before surface impingement. This objective is enhanced by radially-disposed valves, since the injector in a domed firing chamber may be withdrawn further away from the piston. Thus a better match between combustion system and chamber geometry is possible if an enabling technology were available. Again, what is needed in the art is a non-complex means for obviating the restraints of prior art mechanical valve trains to allow radial valving in a domed firing chamber.
3. Inlet-generated swirl of air and fuel is an important combustion control parameter for most diesel engines and some SI engines. The normal prior art method to generate such in-cylinder swirl motion is through the use of one or more “directed” ports wherein the flow direction is generally tangent to the cylinder wall, so that momentum built up in the intake tract is sustained and translated into rotational swirl in the cylinder. This technique may require a relatively long intake tract in the cylinder head in which to develop the necessary momentum, and this in turn can drive the need for a skewed valve layout in the cylinder head (“skewed” as used herein should be taken to mean a layout wherein the valve pairs do not lie in a line parallel to or orthogonal to the axis of the cylinder bank). Such a layout is problematic for a conventional mechanical valve train since the distance from the camshaft to each valve stem is different, resulting in complex linkage solutions or compromised port design. Again, swirl is fundamental to efficient diesel combustion, so the ability to optimize the inlet port for swirl rather than for valve train considerations would provide a competitive advantage. What is needed in the art is a means for removing valve train considerations as port design constraints.
Given these incentives to escape from the constraints of mechanical linkages, several electromechanical and electro-hydraulic concepts have been proposed in publications in the engine arts, but none has been accepted or commercialized to date due to excessive cost, complexity, and durability concerns.
A separate but related interest in the engine arts is variable valve activation (VVA) of engine valves, especially intake valves, also known interchangeably in the art as variable valve deactivation. To selectively shut off one or more engine valves to improve fuel efficiency in low load conditions, various design approaches to partial or total deactivation are well known. In each such design approach, a valve-deactivation strategy is incorporated wherein the rotary motion of an engine cam continues unabated but the lift motion is lost in the translation between the cam and its associated valve(s) by a mechanical decoupling of the valve train. In the prior art, mechanical accommodation is provided for the lost motion via, for example, a variably-latchable rocker arm assembly or a variably-latchable hydraulic valve lifter assembly.
Various electro-hydraulic systems also have been proposed in the prior art, wherein a primary piston actuated by hydraulic linkage to a cam drives a valve stem, and a lost-motion chamber for accumulating hydraulic fluid may be selectively accessed via a high-speed solenoid valve. See, for example, U.S. Pat. No. 6,227,154 wherein a solenoid-actuated three-port spool valve selects between a valve-actuating piston and a lost-motion piston. A general shortcoming of such systems is the flow restriction imposed by the solenoid valve itself, limiting the speed of response of the system and creating high pumping losses. Further, such designs are not readily applicable to non-overhead cam engines. Further, such prior art designs use engine oil as the hydraulic medium, which oil becomes dirty and degraded with carbon deposits during prolonged use, resulting in wear, clogging, and variable performance of the LM system.
An alternative approach is known in the prior art and is exemplarily disclosed in U.S. Pat. No. 4,716,863, wherein a slave piston actuated by hydraulic linkage to a master piston and cam drives an intake valve stem, and a solenoid controls the position of a secondary accumulation piston in a sidearm and “thus expansion of the hydraulic line volume, thereby controlling the opening and closing, timing, and displacement of the intake valve.” Access to the pressure chamber formed in the sidearm does not require passage of hydraulic fluid through a valve. A serious shortcoming of this configuration is that a relatively large, powerful, and expensive solenoid is required to manage precise positioning of the accumulation piston against the entire force brought to bear on the face of the piston; such a solenoid typically lacks the desired high rate of response. The above-recited shortcomings resulting from use of engine oil as the hydraulic medium also pertain.
A further related interest in the engine arts is variation in the timing of opening and closing, and of the amplitude of opening, for both intake valves and exhaust valves for a variety of engine operational modes. This interest extends to both SI and CI engines. When combined with LM capability, the resulting flexibility in valve operation can have very large effects in a wide variety of vehicle and engine parameters, including at least fuel efficiency, ease of starting, low-end torque and turbocharged transient behavior, pollution abatement, vehicle braking; and engine wear, complexity, cost of manufacture, and ease of repair. Comparable improvements in these categories cannot be readily achieved in any way other than VVA/LM.
What is needed in the art is means for efficiently and economically combining variable valve actuation and lost-motion capability in an electro-hydraulic valve train.
It is a principal object of the present invention to provide improvements in fuel efficiency, ease of starting, low-end torque and turbocharged transient behavior, pollution abatement, vehicle braking; and engine wear, complexity, cost of manufacture, and ease of repair in an internal combustion engine.
It is a further object of the present invention to provide such improvements in a compression-ignited engine, whether operating on the Diesel cycle or alternative cycles such as Homogeneous Charge Compression Ignition (HCCI), whether in two-, four-, six- or eight-stroke combustion cycles as are known in the prior art.